Nursing unit arrangements of the general type described herein are known. Typically such units comprise a hollow nursing holder which is adapted at one end to receive a collapsible liquid-retention sac and a nipple. A widely known and used arrangement of this type is the unit described in U.S. Pat. No. 3,075,666 to Hoffstein. It includes a generally cylindrical nursing holder having an annular groove at one end. The groove has on one side a peripheral rim extending from the holder at substantially a right angle and, on the opposite side, a shoulder on the main body portion of the holder. Within the holder, there is disposed a collapsible liquid-retaining sac having the open end turned back and downwardly over the peripheral rim of the holder. A suitable nursing nipple which can be used with such nursing unit arrangements is described in U.S. Pat. No. 2,588,069 to Allen. It has a teat-like portion terminating in a ridge that extends outwardly in the form of a flange having a depending skirt. On the inner surface of the depending skirt an annular groove is provided. The groove is adapted to frictionally engage the peripheral rim of the nursing holder, the portion of the skirt below the groove being seated within the annular groove of the holder. Positioning of the collapsible sac on the holder causes the open end of the sac to stretch over the peripheral rim thereof and resiliently engage the surface of the annular groove of the holder. Positioning the nipple on the holder locks the open end of the sac between the holder and the nipple and the nursing holder, thereby preventing the sac from pulling loose from the holder during use.
Another nursing unit of the general type described is disclosed in U.S. Pat. No. 3,790,017 to Fitzpatrick and Cubitt. It differs from the unit of Hoffstein by providing a nursing holder in which the annular groove proximate to the nipple-receiving end of the holder has an abutment shoulder of rim. This shoulder extends from the groove a lateral distance at least equal to or greater than the thickness of the portion of the nipple which abuts thereon thus providing an arrangement wherein the possibility of accidental or inadvertent removal of the nipple by an infant is substantially reduced. At the same time, the discontinuous shoulder provides ready access by a parent to disengage or unseat the nipple with the tip of the finger or thumb by contact of the finger or thumb with that portion of the skirt of the nipple that is exposed through the discontinuity of the shoulder or rim.
A nursing system having a screw down type of nipple is also known, as disclosed in U.S. Pat. No. 2,624,485, to Boston. Such an arrangement is less likely to result in accidental dislodgment of the nipple by an older infant.
While it is convenient to use a snap-on type of nipple when the infant is young, an even greater securing force is preferable as the infant grows older and reaches the toddler stage and a nursing system employing a screw down type of nipple is more suitable. With the present invention one has the benefit and advantage of both types of systems. This is because the present invention permits extended use of the previously known holders, that is the same holders with older infants or toddlers, thus obviating the necessity of replacing them, while providing greater securing force for the nipple. On the other hand, the present invention permits relatively facile removal of the nipple from the holder by the parent when desired.